This application is based on and claims priority from Japanese Application No. HEI 2-335936 filed Nov. 30, 1990, the disclosure of which is incorporated herein by reference.
The present invention relates to an afocal optical system for use with binoculars and monoculars, particularly to a zooming optical system capable of changing the magnification.
Throughout this specification, the terms "group" and "component" are used interchangeably to refer to a structure comprising at least one lens element.
Eyepiece optical systems such as binoculars are often provided with a dioptry adjustment capability for adjusting the focus in accordance with the eyesight of the user. With zooming optical systems which are capable of adjusting the magnification (focal length), a reference dioptry which does not change with the magnification is determined at the design stage.
If there is even partial overlap between the lens group movable for dioptry adjustment and the lens group movable for zooming, the change in dioptry due to zooming cannot be suppressed for all dioptries. Under these circumstances, a dioptry most commonly used is specified as a reference and the system is designed in such a way that zooming will cause no change in dioptry at least in the case where the system is set for the reference dioptry. Hence, there will be no change in dioptry due to magnification alterations if the system is set for the reference dioptry. However, if the system is set for values other than the reference dioptry, magnification alterations will cause changes in dioptry, and another dioptry adjustment becomes necessary.
FIG. 15 shows a conventional zooming optical system. An objective optical unit that is positioned closer to the object than a field ring S is composed of a fixed first lens group G1 and a second lens group G2 movable for zooming, whereas an eyepiece optical unit positioned closer to the eye than the field ring S is composed of a third lens group G3 movable for zooming and a fourth lens group G4 that is fixed during zooming. Dioptry adjustment is effected by moving altogether the three lens groups enclosed with a dashed line, i.e., the second lens group G2, the third lens group G3 and the fourth lens group G4.
A problem with this prior art zooming optical system is that the lens groups movable for dioptry adjustment experience a great change in focal length during zooming, since those dioptry adjusting lens groups contain all of the lens groups G2 and G3 which have a zooming capability. The eyesight mismatch decreases with the decreasing change in the focal lengths of the dioptry adjusting groups, so a great eyesight mismatch will inevitably occur in the conventional system.